This invention relates to a dynamic bridge system with a common mode range extension and more particularly to such a system with noise gain reduction.
A dynamic bridge or subtractor circuit employs an amplifier and bridge to suppress common mode signals and pass or amplify normal mode signals. See Operational Amplifiers Theory and Practice, James K. Roberge, John Wiley and Sons, Inc., pages 449-450. Such circuits are often used where the common mode signals are substantially larger than the normal mode signals. In some cases the common mode signals are so large that they exceed the power supply limits by a significant amount. One approach to this problem uses a pair of balanced loads at the inputs to the amplifier to reduce the common mode signals. See High Common-Mode Voltage Difference Amplifier, Analog Devices Tech. Notes, Analog Devices, Inc., Norwood, Mass., pages 1-12. While this works well to reduce the common mode signals while maintaining the normal mode signals it introduces a further problem in that it increases the noise gain of the circuit. See Analog Dialogue, Noise and Operational Amplifier Circuits, Lewis Smith and D. H. Sheingold, A Journal for the Exchange of Analog Technology, Vol. 3, No.1, pages 3-14. A common practice is to make the balanced loads small enough to effect sufficient attenuation of the common mode signals applied to the amplifier while at the same time large enough to minimize the noise gain. Often this is not satisfactory and provides less performance than desired.
It is therefore an object of this invention to provide an improved dynamic bridge system with an extended common mode range.
It is a further object of this invention to provide an improved dynamic bridge system with noise gain reduction.
It is a further object of this invention to provide an improved dynamic bridge system with increased common mode rejection.
The invention results from the realization that a dynamic bridge system with extended common mode range and rejection and noise gain reduction can be achieved by having a pair of balanced loads connected to the intermediate terminals of the bridge circuit along with the inputs to the differential amplifier and using an inverting amplifier responsive to the common mode signals at the inputs of the differential amplifier for driving the other ends of the balanced loads in opposition to changes in the common mode signals at the input terminals of the bridge circuit.
The invention features a dynamic bridge system with common mode range extension including a dynamic bridge circuit having a pair of input terminals for receiving common mode and normal mode signals, a pair of intermediate terminals, an output terminal and a reference terminal. There is a differential amplifier having its inputs connected to the intermediate terminals and its output connected to the output terminal. A pair of balanced loads is each connected at one end to an intermediate terminal. An inverting amplifier responsive to the common mode signal at the inputs of the differential amplifier drives the other ends of the balance loads in opposition to changes in the common mode signals at the inputs of the differential amplifier.
In a preferred embodiment, the system may include a feedback attenuator circuit between the output of the differential amplifier and the output terminal for increasing the normal mode gain of the system. There may be a compensating circuit in series with the reference terminal for balancing the reference terminal impedance relative to the output terminal impedance. There may be a gain adjustment impedance interconnected between the attenuator circuit and the compensating circuit for adjusting the gain of the system without disturbing the balance of the bridge circuit. The inverting amplifier may include at least a transistor and a load for setting the limits of the inverting amplifier output to match the limits of the common mode signal at the input terminals